The present invention relates to a composite electrode for use in electrochemical cells, and more particularly, to a composite electrode for use in fuel cells, such as hydrogen/oxygen fuel cells.
Certain fuel cells convert chemical energy into electrical energy by reacting different gases at electrocatalytic surfaces on anode and cathode electrodes which are positioned on opposite sides of an ion exchange membrane. Generally, the gas introduced to the anode is categorized as a fuel while the gas introduced to the cathode is an oxidant. Utilizing hydrogen/oxygen solid electrolyte fuel cells as illustrative, hydrogen is introduced via a gaseous stream to the anode side of an ion exchange membrane and is electrochemically oxidized in the presence of a suitable catalyst, such as platinum or a platinum alloy, in accordance with the following general reaction: EQU H.sub.2 =2H.sup.+ +2 electrons (1)
Oxygen is introduced to the cathode side of the ion-conducting membrane in a second gaseous stream and, is electrochemically reduced in accordance with the following general reaction: EQU O.sub.2 +4H.sup.+ +4 electrons=2H.sub.2 O (2)
Conventionally, fuel cells are designed by positioning an ion exchange membrane within a reactant chamber to define an anode compartment on one side of the ion exchange membrane and a cathode compartment on the other side of the ion exchange membrane. Electrodes are postioned in each compartment and the chamber is equipped with suitable manifolds for the introduction of gaseous streams into the anode and cathode compartments. The reactant chamber also has outlets for product water and any unreacted gases. The ion exchange membrane is a permselective ion transporting membrane, i.e., selectively transports only anions or cations depending upon the charge of groups bound within the polymeric matrix of the ion exchange membrane. In a hydrogen/oxygen fuel cell, protons liberated during electrochemical oxidation of hydrogen are selectively transported through the ion exchange membrane to the cathode. Traditionally, the electrodes used in fuel cells have been coated with or are comprised of an electrocatalyst, such as platinum or a platinum containing compound, to increase the rate of the electrochemical oxidation and reduction reactions occurring in the anode and cathode compartments, respectively. The electrons generated in the anode compartment are collected by a current collector and are transported through an external circuit which contains a load to the cathode compartment. Current collectors can be constructed of any suitable electrically conductive material, such as any stable metal or a carbon black. Often, the material of which the reactant chamber is constructed functions as the current collector. As conventionally constructed, fuel cells can operate at approximately 60% efficiency, i.e., convert approximately 60% of the available chemical energy in the reacting fuel to electricity while the remaining 40% is converted to thermal energy.
Fuel cell operation involves three separate modes of transport. First, the introduction of reactant gases into the fuel cell, for example, a hydrogen/oxygen fuel cell, from a supply source and movement of these gases through the fuel cell during operation involves molecular transport. Also, movement of water formed as a result of the electrochemical reduction of oxygen in accordance with reaction (2) within the hydrogen/oxygen fuel cell involves molecular transport. Secondly, transport occurs as protons, i.e., hydrogen ions, produced as a result of reaction (1), move from the anode compartment electrode, through the ion exchange membrane, and into the cathode compartment electrode of the fuel cell where electrochemical reduction occurs. Lastly, electrons generated in accordance with the electrochemical oxidation reaction (1) are transported via a current collector through an external conductive path having an electrical load to the cathode compartment to serve as a reactant for the electrochemical reduction of oxygen. Fuel cell performance is reduced by any internal impedance to any one of the three modes of transport which occur during operation thereof.
In an effort to reduce the internal resistance to transport, particularly proton and electron transport, and thereby increase fuel cell performance, fuel cells designs have evolved to a "zero-gap" configuration wherein the reaction chamber, electrodes and ion exchange membrane are positioned in a continguous relationship. To achieve a "zero-gap" configuration and the attendant reduction in internal resistance to transport, conventional electrode structures, such as a wire mesh screen coated with an electrocatalyst, have been embedded on each surface of the ion exchange membrane. Additionally, composite electrodes of ion exchange polymers, metals or carbon compounds, and electrocatalytic compounds have been utilized in an effort to construct an electrode wherein an electron conductor, a proton conductor, and an electrocatalyst are incorporated into a three-phase interface to minimize internal resistance to transport.
Solid ion exchange polymers having fixed anionic sites in the form of sorbed anions or chemically bonded anions have been used to form the permselective ion exchange membrane used in fuel cells. Recently, polymers possessing sorbed or grafted ions of strong proton acids, for example, sulfonic or phosphonic acids, have been employed as ion exchange membranes in fuel cells due to the relatively rapid proton transfer thereof. Nafion.RTM., a perfluorosulfonic acid membrane, has gained increasing popularity as an ion exchange membrane for fuel cells. Typically, the ion exchange membrane is constructed of a relatively thin, e.g., 0.002-0.012 in. thick, sheet or film of an active ion exchange polymer. Several separate sheets or films of active ion exchange polymers, each of which possesses different conducting and/or wetting properties, can be thermally laminated to form a unitary ion exchange membrane for use in fuel cells.
With the advent of the unitary, relatively thin ion exchange membranes and electrode assemblies, fuel cells have been arranged in a stacked configuration. As such, the plate or housing separating the anode and cathode compartments of two adjacent fuel cells electrically connects the fuel cells in series by conducting electrons generated in the anode of one fuel cell directly to the cathode of an adjacent fuel cell. This common plate is referred to as a "bipolar plate" and additionally serves as a positive barrier to prevent mixing of anode and cathode gas flows between adjacent cells. Thus, the relatively low voltage generated by a single fuel cell, 0.5-0.9V, can be added in series to obtain useful voltages of, for example, 120V.
As previously mentioned, composite electrodes comprising a homogeneous mixture of an ion exchange polymer, an electrical conductor, and an electrocatalyst have been supported for use in fuel cells. The ion exchange polymer, for example, Nafion.RTM., serves the function of a composite binder and an electrolyte for conducting cations. An electrical conductor, such as carbon powder, functions not only to conduct electrons but also as a catalyst support. These composites are formed with a filler material which is subsequently removed by application of specific aqueous solutions or heat to provide a porous electrode matrix which allows gaseous reactants to flow therethrough. Accordingly, an electrode matrix structure is provided which forms a conductive skeleton for transport of both electrons and protons relatively uniform throughout the entire electrode. However, loading catalyst throughout the proposed composite electrode significantly increases the amount of catalyst in the electrode thereby promoting fuel cell catalyst inefficiency since only a portion of the loaded catalyst is utilized in the electrochemical reactions.
Accordingly, it is an object of the present invention to construct a composite electrode for use in electrochemical cells in which substantially all of the electrocatalyst used in fabricating the electrochemical cell is positioned only along a zone within the electrode where the transparent rates of electrons and protons are approximately equal.
Another object of the present invention is to provide a composite electrode for use in electrochemical cells which is constructed to have increasing electronic conductivity from the catalyst loading zone to a current collector on one face of the electrode, and to have increasing protonic conductivity from the zone of catalyst loading to the face of the electrode which engages the ion exchange membrane of the electrochemical cell.
It is a further object of the present invention to provide a composite electrode for use in electrochemical cells wherein the amount of electrocatalyst necessary to achieve the highest electrochemical cell performance, as measured by the value of watts per mg of electrocatalyst, is minimized.
It is still a further object of the present invention to provide a composite electrode for use in electrochemical cells wherein the electrocatalyst is more efficiently utilized, as measured by the voltage of a single electrochemical cell.